1. Field of the Invention
The present invention relates generally to the field of medical devices. In particular, this invention pertains to a stylet that helps increase the rigidity of a medical device, particularly when the medical device is being inserted.
2. Description of Related Art
Numerous types of catheters are well-known in the prior art; for example, peripherally inserted central catheters (PICCs) are commonly used in the prior art. Peripherally inserted central catheters are used to access the vascular system. In particular, PICCs are used when long-term-vascular access is required (e.g., more than two weeks indwelling time in a patient) to avoid multiple injections of the patient. A PICC is a long catheter that is often inserted mid-way on a patient""s arm and the end of the catheter is often placed in the superior vena cava of the heart. PICCs often provide access to a patient""s vascular system for chemotherapy or other types of intravenous medication. Because the PICC must often travel through many natural obstacles in the venous/vascular system, the catheter must be soft and pliable for ease of navigation and to avoid trauma to a blood vessel. In addition, the catheter must also be made of bio-compatible materials so that it will not irritate a patient""s veins during the catheter""s long dwelling time.
Unfortunately, PICC lines made of a soft bio-compatible material are very difficult to insert into a patient because its soft or pliable construction causes the PICC to often collapse and bend before entering the patient""s veins. In order to more quickly and efficiently insert a PICC, a flexible metal stylet (or guidewire) is used as a stiffening member inside the catheter during insertion. Once the catheter is placed inside the patient, the guidewire is removed. Guidewires, however, cause several problems. There is increased friction between the guidewire and the inner wall of the catheter since the catheter often makes many turns and twists before reaching its final destination. As a result, it is often difficult to manipulate and then remove the guidewire. Second, the use of the guidewire also introduces the possibility that the guidewire may perforate a catheter when it is forced against the catheter.
One solution to these problems is to flush the catheter before using it, during the insertion process to aid manipulation of the guidewire, and/or after the insertion process to assist removal of the guidewire. These flushing solutions often contain heparin and saline. The need for a flushing system and a stiffening member, such as a stylet, becomes even more significant as catheters decrease in diameter. A smaller catheter is often more beneficial for patients with small or fragile veins since a smaller catheter causes less trauma to the patient""s veins. The use of smaller catheters has also necessitated that the guidewire or stylet be smaller. Yet stylets have not decreased in size to the degree needed. Consequently, the fluid flow area for the flushing solution is decreasing at the same time the need for the flushing solution and a larger fluid flow area are increasing. This is counter-productive.
Referring to FIG. 1, a cross-section of a prior art guidewire 102 is shown in a small catheter 100 with minimal space for the fluid flow area 104. In an exemplary catheter with a 0.035 inch diameter, a cross-sectional area of guidewire 102 occupies about 70% of the cross-sectional area of the lumen of catheter 100. Consequently, only about 30% of the cross-sectional area of catheter""s 100 lumen is available as the fluid flow area 104. Another prior art embodiment of a guidewire 200 is shown in FIG. 2A. The guidewire 200 is made of three elements: a flat ribbon of material 207 adjacent to a core 209, both of which are disposed within a twisted-helical coil 205. The resulting fluid flow area 203 between the guidewire 200 and the catheter 201 is minimal.
A side view of the guidewire 200 of FIG. 2A is illustrated in FIG. 2B. The helical coil 205 is welded on both ends of the ribbon 207 at 211a and 211b. The twists of the helical coil 205 are often not visible to the naked eye, but for the sake of clarity, the space between each of the coils in coil 205 have been exaggerated in FIG. 2B. The coil 205 often flexes as it twists and turns with the catheter 201. This allows some fluid to flow in between the coils 205 and around the core 209, which provides stiffness, and the flat ribbon 207. In addition, the core 209 can also be welded to the flat ribbon 207.
FIG. 3 illustrates a cross-section of another prior art embodiment in which the guidewire 301 is made of three separate wires 301a-c, which are braided together. Each wire may have a diameter of approximately 0.008 inches while the internal diameter of the catheter 300 may be 0.032 inch. FIG. 3 illustrates one disadvantage of the prior art systems. The length of the guidewire cannot be changed without altering or compromising the structural integrity of the guidewire. For example, guidewire 301 of FIG. 3 cannot be cut without compromising or destroying the structural integrity of the guidewire 301 as its three wires 301a-c will unravel and separate. The same is true for the guidewire 200 of FIG. 2 and its three components, 205, 207 and 209.
As is evident from FIGS. 1, 2 and 3, the fluid flow area (104, 203, 303) around each respective guidewire (102, 200, 301) is severely restricted as the diameter of the guidewire approaches the internal diameter of the catheter. Many prior art guidewires are made of metal so that the guidewire may not be cut when adjustability of the catheter length is desired. As a result, in these prior art embodiments, the guidewire is adjusted by first withdrawing the guidewire from the proximal end of the catheter and then cutting the catheter. These catheters thus require extra-tedious manipulations before being able to use them.
Smaller fluid flow due to decreased area between the guidewire (or stylet) and the inner wall of the catheter is undesirable. Adequate fluid flow is necessary for the flushing solution to aid insertion and removal of the guidewire (or stylet) in the catheter. In addition, unhindered fluid flow helps prevent friction between the guidewire (or stylet) and the inner wall of the catheter. Thus, it is desirable that a guidewire (or stylet) serves as a stiffening member in a medical device, such as for example, a PICC catheter, during insertion of the medical device. It is also desirable that the shape of the stiffening member helps increase fluid flow around the guidewire (or stylet) disposed within the medical device.
The present invention describes a stiffening member that allows an increase in the fluid flow area within a medical device, such as a catheter. In one embodiment, the stiffening member is an elongated, substantially rectangular stylet that is disposed within the catheter, such that the stylet can occupy less than half of the lumen area of the catheter. As a result, adequate fluid flow is allowed between the stylet and the inner wall of the catheter. This increases patient comfort. In a preferred embodiment, the stylet is made of substantially flat stock such that in cross-section, the longer dimension of the stylet closely approximates or is slightly smaller than the internal diameter of the catheter. Furthermore, in the preferred embodiment, the flat stock is twisted over the length of the stylet to achieve equal rigidity or bendability over the entire length of the stylet in any axis. Different pitches (i.e., number of twists or turns per inch) may be used to obtain stylets with different degrees of rigidity. Moreover, the pitch within a single stylet may be varied to create multiple stiffnesses within a single stylet. For example, the proximal end can have a greater pitch than the digital end of a stylet to create a softer proximal end. This is beneficial since the proximal end is inserted first into the patient.
The elongated, substantially rectangular stylet comprises the majority of the cross-sectional area of the substantially rectangular stiffening member. Substantially rectangular, as defined, can also include a cross-sectional area of a single element that is shaped like various versions of an I-beam, a dog-bone, an ellipse, an oval, and a rectangle. The cross-section of the substantially rectangular stylet has a long first axis and a short second axis that is not aligned with the first axis. The first and second axes, however, are coupled together and are part of a single element stylet.
The cross-sectional area of the stiffening member is usually such that one dimension (the xe2x80x9clongxe2x80x9d dimension) is at least twice or most preferentially, eight times the length of the other dimension (the xe2x80x9cshortxe2x80x9d dimension); in this way, the stiffening member is substantially rectangular at least in cross-sectional view. This allows the flushing fluid to flow through at least half of the lumen area within the catheter, and more preferably through about 80% of the catheter""s lumen area.
In another embodiment, a cross-sectional area of the elongated stylet is a percentage up to 65% of a cross-sectional area of the catheter""s lumen. The available fluid flow area is the area not occupied by the elongated stylet within the catheter""s lumen. The cross-sectional area of the elongated stylet is formed substantially by a single stiffening member. In yet another aspect of the present invention, a coating can be placed around the single stiffening member. The coating does not substantially decrease the fluid flow around the stylet. Moreover, the coating can be selected from a group consisting of a hydrogel, or Teflon(trademark), which applies to a tetrafluorethylene fluorocarbon polymer and a fluorinated ethylene-propylene resin.